Animation: How HIV Causes DiseaseCD4 T-cells (a type of white blood cell) are important to the body's defence against infections. This animation describes how the immune system is weakened by the HIV virus, which targets CD4 T-cells and leads to their gradual decline in number. Low to very low levels of CD4 cells put patients at risk for 'opportunistic infections' that take advantage of the body's weakened immune system.